This invention relates to a linear position detection device and, more particularly, to an induction type detection device capable of changing coefficient of induction in response to relative displacement of a core with respect to a coil thereby to obtain an output signal corresponding thereto and, more particularly, to a linear position detection device capable of producing an enhanced change in the coefficient of induction by using both reluctance change (permeance change) and eddy current loss as parameters thereby realizing a high-precision detection.
The invention relates also to a linear position detection device capable of obtaining a change in the coefficient of induction using at least the eddy current loss as a parameter. The invention relates also to a position detection device for a piston rod of a fluid powered cylinder and, more particularly, to an induction type piston rod position detection device capable of changing the coefficient of induction in response to relative displacement of a piston rod with respect to a winding fixed to the cylinder main body thereby to produce an output signal corresponding thereto and, more particularly, to a piston rod position detection device provided with a relatively conductive substance portion such as copper about the piston rod and being capable of producing an induction coefficient change using at least the eddy current loss as a parameter by the action of this conductive substance portion.
Known in the art of a linear position detection device of a variable reluctance type is a differential transformer. There are also disclosed detection devices of a phase shift system employing a variable reluctance type detector, for example, in Japanese Preliminary Utility Model Publication No. 135917/1982 or No. 136718/1983 or the U.S. patent application Ser. No. 06/348,674 (now U.S. Pat. No. 4,556,886) or West Germany Patent Application No. P3205032.1. Such prior art detection device are all constructed in such a manner that the coefficient of induction of the coil is caused to change only in response to reluctance change due to displacement of a magnetic member with a resulting limitation posed on the accuracy of detection. Particularly, the accuracy of detection for a minute displacement is limited due to the mechanical structure of these devices.
This will be explained in more detail with reference to FIG. 1. A prior art variable reluctance type linear position detector 10 comprises primary coils 1A-1D and secondary coils 2A-2D relatively fixed at predetermined locations and a core section 3 which is inserted in the coil space in such a manner that it is capable of a relative linear displacement. The core section 3 consists of a plurality of magnetic cores 3a disposed at a predetermined interval in the linear displacement direction (the direction of arrows L,L) and non-magnetic spacers 3b interposed therebetween. For example, the cores 3a are of a cylindrical configuration with a length of about P/2 and the spacers 3b are also of the length of P/2. The coil operates with four phases which are distinguished, for convenience's sake, by reference characters A, B, C and D. The reluctance produced in each of the phases A, B, C and D in response to the relative position of the cores 3a with respect to the coils is 90.degree. out of phase with that produced in each adjacent phase. The locations of the respective coils and the size and configuration of the cores 3a are so determined that if, for example, the phase A is a cosine phase, the phase B becomes a sine phase, the phase C a minus cosine phase and the phase D a minus sine phase.
In the example of FIG. 1, the primary coils 1A-1D and corresponding ones of the secondary coils 2A-2D are wound in the same location with the coil length of the respective coils being about P/2 (P being any number). The coils 1A, 2A of the phase A and the coils 1C, 2C of the phase C are provided adjacent to each other and the coils 1B, 2B of the phase B and the coils 1D, 2D of the phase D are also provided adjacent to each other. The interval between the coil group of the phases A and C and the coil group of the phases B and D is "P(n.+-.1/4)" (n being any natural number).
The coils 1A, 2A, 1C and 2C of the phases A and C are received in a cylindrical case 4 made of a magnetic substance such as iron and the coils 1B, 2B, 1D and 2D of the phases B and D are also received in a similar cylindrical case 5. These iron cases 4 and 5 function to prevent crosstalking between the respective coils and also increase the union of the magnetic circuit.
In a case where an output signal is to be obtained by the phase shift system, the coil group of the phases A and C whose phases of reluctance change are opposed to each other is excited by a common primary AC signal (for example, a sine wave signal) to produce a differential output and the coil group of the phases B and D whose phases of reluctance change are opposed to each other is excited by a common AC signal which is out of phase with the above mentioned AC signal by a predetermined angle (for example, a cosine wave signal) to produce a differential output. As a result, as a sum signal of the differential outputs of the phases A and C and the phases B and D, a signal which is equivalent to a signal being phase-shifted in the electrical angle of the primary AC signal by a phase angle corresponding to the linear position of the core section 3 (a linear displacement amount within the distance P) is obtained.
In the detection device as described above, the reluctance of each phase is determined in response to the amount of the relative displacement of the cores 3a and in this determination, a fixed value which is determined by a gap between the core section 3 and the coil section participates as an offset amount of reluctance of the magnetic circuit of each phase. On the other hand, resolution of detection for a minute displacement can be increased by shortening the interval P of the cores 3a. However, the fixed gap amount cannot be reduced in proportion to the reduction of the interval P and, besides, limitation is posed on such reduction by the mechanical structure of the device. Accordingly, the attempt to increase the resolution of detection by reduction of the interval P is likely to result in relative decrease in the width of reluctance change with respect to the displacement of the cores 3a, i.e., the ratio of the width of level change corresponding to the displacement to the secondary side induced voltage level and hence failure in obtaining an adequate accuracy.
It is therefore a first object of the present invention to increase the accuracy in detection in a linear position detection device of a type in which the reluctance change is produced in response to the displacement. It is another object of the invention to reduce the size of the device thereby to obtain sufficient accuracy in case the device is adapted for detection of a minute displacement.
A differential transformer in which a conventional iron core is replaced by a core made of a weak magnetic substance such as copper or aluminum is disclosed in Japanese Preliminary Patent Publication No. 1210/1982. This utilizes reluctance change due to the eddy current loss produced by a good conductive and weak magnetic metal in the magnetic field instead of the reluctance change responsive to the displacement of the strong magnetic core such as iron. This prior art eddy current type differential transformer cannot be employed, as the conventional iron core type differential transformer, for detection of a linear position other than that within a relatively short range. Further, since the detected linear position is estimated by the magnitude of the output analog voltage level, an erroneous operation is likely to take place due to disturbance and noise.
On the other hand, Japanese Preliminary Utility Model Publication No. 136718/1983 discloses, as a linear position detection device of a variable reluctance type, a phase shift type device in which a linear position is estimated by an electrical angle of an output AC signal.
In this device, magnetic substance cores of predetermined length are arranged at a predetermined interval and the linear position can be detected over a relatively long range. However, this device requires processing and assembling of a large number of magnetic substance cores and hence it is difficult to reduce the manufacturing cost.
It is therefore a second object of the invention to achieve, in a linear position detection device capable of producing change in the induction coefficient, detection of the linear position over a relatively long range and also detection of the linear position by a phase-shift system. It is another object of the invention to provide such linear position detection device at a relatively low cost.
U.S. Pat. No. 3,956,973 discloses a piston rod position detection device in which an annular or spiral groove is formed in the iron or other magnetic substance metal about the piston rod and a transducer which produces an electric pulse signal in response to entry of the groove accompanying the displacement of the piston rod is fixedly provided on the cylinder main body.
European Laid-open Patent Publication No. 0115008 discloses a piston rod position detection device in which magnetic rings are provided about a piston rod at a predetermined interval, primary windings and secondary windings of plural phases are provided on the cylinder main body, the primary windings of the respective phases are individually excited by AC signals which are out of phase from one another and an AC signal which is phase-shifted responsive to the position of the piston rod is produced on the secondary windings, and the amount of the phase shift is digitally counted whereby the piston rod position can be detected absolutely.
The above European Laid-open Patent Publication also discloses a piston rod position detection device in which a stator with a plurality of poles wound with primary and secondary windings is fixedly provided on the cylinder main body, a plurality of projections are provided on a magnetic substance portion about the piston rod, the primary windings of the respective phases are individually excited by AC signals which are out of phase with one another and an AC signal which is phase-shifted responsive to the piston rod position is obtained in the secondary windings, and the amount of the phase shift is digitally counted whereby the piston rod position can be detected absolutely.
Since the detection device disclosed in the above-mentioned U.S. Pat. No. 3,956,973 simply produces a pulse signal in response to entry of the groove formed in the piston rod, a construction for counting this pulse must be employed for obtaining this pulse signal which substantially results in an incremental pulse generation and counting system. Accordingly, this device has the defect that it cannot detect the piston rod position absolutely. Besides, the forming of the groove necessitates machine working of the piston rod which is a rather troublesome process.
The detection device disclosed in the above described European Laid-open Patent Publication is of an absolute system so that it has no problem described above concerning the absolute detection. However, the problem of requiring the troublesome processing or assembling of the magnetic rings or projections about the piston rod still remains unsolved in this device also.
Further, since all of the prior art devices are so constructed that the change in the induction coefficient can be produced by the presence or absence of the groove or projection of iron or magnetic metal, the depth of the groove or the height of the projection must be of a sufficient magnitude for securing a sufficient accuracy.
It is therefore a third object of the invention to provide a piston rod position detection device capable of solving the above described various problems.